1. Field of the Invention
This invention relates to butterfly type valves, and particularly to installed peripheral seals therein.
2. Description of the Prior Art.
Butterfly type valves for use in controlling the flow of high temperature fluids make use of metallic piston type split ring seals recessed into a groove in the valve periphery. A seal assembly as installed in a valve groove may comprise two or more split rings in a face to face relation substantially filling the groove between what may be regarded as front and back edges thereof. An outer periphery of the seal assembly projects from the groove for a sealing engagement with a valve accommodating bore wall. An inner periphery terminates within the groove.
The valve construction of the prior art has certain inherent leak paths including that between the seal outer periphery and the bore wall where surface irregularities may exist. Also, upstream fluid pressure can reach the groove interior, as a consequence of the seal assembly being displaced against the back wall of the groove, and from there migrate through ring gaps and between ring interfaces to the downstream side of the valve.
In the prior art, in dealing with the leakage problem, any one of a variety of techniques has been adopted, or combinations thereof. According to one thereof, the natural tendency of the ring or rings to expand is enhanced so that the seal pushes tightly against the bore wall. This has the disadvantage of promoting excessive valve wear, and does nothing to deal with the problem of leakage between ring interfaces. The number of seal rings can be increased but this adds to cost, and, again, does not deal with interfacial leakage. Another common, but costly, recourse is critical machining to insure precise fits, very fine surface finishes and close tolerances. Frequently, and despite practice of the best prior art techniques, the amount of fluid leakage cannot be reduced to values a given application may require, making use of a butterfly type valve in that application impractical.